Immunochromatographic strip formats have become increasingly popular for qualitative and semi-quantitative assays which use visual detection schemes. This type of assay involves the application of a liquid test sample suspected of containing an analyte to be detected to an application zone of an immunochromatographic test strip. The strip is comprised of a matrix of absorbant material through which the test fluid and reagents for detecting the analyte can flow by capillarity from the strip""s application zone to a capture zone where a detectable signal, or the absence thereof, reveals the presence of the analyte. Typically, the strip will include means for immunospecifically binding the analyte to be detected with its specific binding partner which bears the detectable label. In one such scheme, the strip contains an enzyme labeled, mobile binding partner for the analyte which is located in a zone downstream from the sample application zone. If analyte is present in the test sample, it will combine with its labeled binding partner to form a complex which will flow along the strip to a detection zone which contains a substrate for the enzyme label which is capable of providing a colored response in the presence of the enzyme. The strip may contain a zone in which the analyte is immobilized, so that labeled binding partner which does not combine with analyte, due to the absence of analyte in the sample, will be captured and thereby inhibited from reaching the detection zone. There have been various modifications of this technique, all of which involve some competitive specific binding system in which the presence or absence of analyte in the test sample is determined by the detection or lack thereof of labeled binding partner in the detection zone.
An alternative to the above described immunometric assay which detects the free labeled binding partner is the so called sandwich format in which the capture zone contains immobilized antibodies against an epitope of the analyte which is different from the epitope to which the labeled antibody is specific. In this format, there is formed a sandwich of the analyte between the immobilized and labeled specific binding partners and it is therefore an immunometric assay which detects the bound, labeled specific binding partner.
Not all of the schemes for immunochromatography rely on an enzyme labeled binding partner/enzyme substrate for providing the signal for detection of the analyte. In U.S. Pat. No. 4,806,311 there is disclosed a multizone test device for the specific binding assay determination of an analyte and an immobilized binding partner therefore together with a capture zone for receiving labeled reagent which migrates thereto from the reagent zone. The capture zone contains an immobilized form of a binding substance for the labeled reagent. The labeled reagent bears a chemical group having a detectable physical property, so that it does not require a chemical reaction with another substance in order to be detected. Exemplary of such groups are species of fluorescers, phosphorescent molecules, radioisotopes and electroactive moieties.
U.S. Pat. No. 4,703,017 describes the use of visible particulate labels for the receptor. Various particulate labels such as gold sol particles and visible dye containing liposomes are mentioned. In WO-96/34271 there is disclosed a device for determining a target analyte and creatinine in a fluid test sample which device has an assay strip for the detection of creatinine and a second assay strip for the detection of the target analyte. The creatinine concentration can be determined colorimetrically or by the specific capture of labeled creatinine binding partners. The concentration of the target analyte is corrected based on the sample""s creatinine concentration which correction can either be done manually or by means of a properly programmed reflectance analyzer.
Immunochromatographic strip formats provide a viable system for the determination of various analytes (whether they be antigens or antibodies) but suffer from the limitation that they yield results which are at best semi-quantitative when, for some analytes, more precise, quantitative results are required. One variable which needs to be controlled in analyses using immunochromatographic strips is temperature control. Temperature is an important variable because all immunochemical reactions are characterized by two temperature dependent opposite reactions at the same time. These are the formation of an immune complex from an antigen and its antibody and the appearance of free antigen and antibody by dissociation of the immuno complex. Increasing the temperature increases the rate of reaction, and because immunochromatic strip formats are usually measured under non-equilibrium conditions due to the short assay times involved, temperature control, both within and between laboratories, is critical for insuring consistent reaction rates thereby providing more reproducible assay quantitation. Currently, temperature is not controlled. Typically immunochromatographic strips are run at ambient temperature which can range from 20-30xc2x0 Centigrade. Using the rule of thumb that reaction rates double for every 10 degree centigrade increase in temperature, it is apparent that controlling temperature allows for control of the immunochemical reaction thereby leading to more reproducible results.
Various means for controlling temperature in conjunction with analytical devices are available. In U.S. Pat. No. 5,221,448 there is disclosed an electrophoresis instrument including a capillary tube in an air-cooled cartridge. The cartridge also supports a spherical lens which is part of the optical detection apparatus. The cartridge rests in a manifold which includes the sample and buffer reservoirs. Measuring the electrical resistance of the capillary tube during the electrophoresis process controls the temperature of the capillary tube and cooling or heating of the cartridge is accomplished by circulating temperature controlled air over the tube.
In U.S. Pat. No. 5,232,667 there is disclosed a temperature control system for a disposable cartridge including a sample chamber in which a medical diagnostic device or other electrochemical analytical device is enclosed. The disposable cartridge may include its own heating element on a sensor chip and plugs into a terminal which contains electrical input/output connections. The outer surface of the chip is exposed and a remote temperature sensor, which senses the temperature of the outer surface of the chip of the measuring cell and generates a control signal, is used with conventional temperature control circuitry as the basis for thermostatic control of the cell temperature.
U.S. Pat. No. 4,847,470 discloses an apparatus for warming blood from storage to physiologic temperatures at transfusion rates up to 160 milliliters per minute and includes a flat metal cartridge formed by a pair of thin, generally rectangular, planar members spaced slightly apart in parallelism and sealed at their peripheral edges to define one or more thin, constant width and uniform in thickness ribbon like conduits through which blood flows from an inlet port to an outlet port at opposite ends of the cartridge.
The present invention is a dry assay device for determining the concentration of at least one analyte in a fluid test sample. The device comprises a strip of absorbant material through which the fluid test sample can flow which strip has a region containing specific binding partners for the analyte which binding partners are marked with a detectable label and a separate detection region for the labeled binding partners. The invention comprises an improvement which involves placing the strip in a hollow casing constructed of a fluid test sample impervious solid material having a top and a bottom which when mated provide a hollow chamber which chamber is in fluid communication with the exterior of the casing and which casing provides an aperture through which the detection region can be observed from outside the casing. The casing contains a thermally conductive material in thermal communication with the strip of absorbant material.
Also included within the scope of the present invention is the method of carrying out an immunochromatographic assay using the device described above.